Bilirubin Calculation and Measurement

Understanding Bilirubin Types

Bilirubin exists in two main forms: unconjugated and conjugated, with unconjugated bilirubin being insoluble and transported to the liver, where it is converted to soluble conjugated bilirubin for excretion 1, 3
Most laboratories routinely report total bilirubin, which comprises both unconjugated and conjugated fractions 1
The terms "direct" and "conjugated" hyperbilirubinemia are often incorrectly used interchangeably, with direct bilirubin including both the conjugated fraction and delta bilirubin (bound to albumin) 4

Unconjugated Bilirubin can be calculated using the formula: Unconjugated Bilirubin = Total Bilirubin - Direct Bilirubin 1
Most clinical laboratories will report both total bilirubin and direct bilirubin, allowing for the calculation of unconjugated bilirubin 1

In patients with Gilbert's syndrome, a common cause of isolated elevated bilirubin, unconjugated bilirubin should be less than 20-30% of the total bilirubin in the absence of hemolysis 4, 5
The presence of delta bilirubin (bound to albumin) can cause direct hyperbilirubinemia to persist longer due to its 21-day half-life 4

In neonates, physiological increases in total bilirubin are primarily unconjugated 1, 6
When evaluating hyperbilirubinemia in clinical trials for cholestatic liver diseases, diagnosis of Gilbert's Syndrome is confirmed by calculating the amount of conjugated bilirubin, which should be less than 20%-30% of the total bilirubin 4, 7

Do not subtract direct-reacting (conjugated) bilirubin from total bilirubin when using guidelines for phototherapy and exchange transfusion in neonates 8
In cases where direct bilirubin is 50% or more of total bilirubin, standard calculation methods may not be reliable, and expert consultation is recommended 8